The present invention relates to a device for removing excess pigment from the surface of the image carrier of a non-impact printing machine.
In present day equipment used for data processing, greater and greater use is being made of high-speed printers in which the characters are printed without relying on raised printing characters impacting on a recipient sheet of paper. Such printing machines, which are termed non-impact or again strike-less transfer machines, generally have an image carrier which is usually formed by rotary drum or an endless belt on the surface of which it is possible, by electrostatic or magnetic methods, to form sensitized zones which correspond to the characters to be printed and which are capable of attracting solid particles of pigment. This image carrier is then brought into contact with a recipient sheet of paper to enable the solid particles held by the sensitized zones of the carrier to be transferred to the sheet so that they can be permanently affixed to it.
To apply solid particles of pigment to the image carrier of a printing machine of this kind, use may be made of various applicator arrangements, such, for example, as that which is incidentally described and illustrated in U.S. Pat. No. 3,161,544. However, despite the care which is taken in producing such applicator arrangements, it is very difficult to prevent the particles of pigment from not only depositing in excessive quantities on the sensitized zones of the image carrier but also, even though in very small amounts, from depositing outside these areas. This phenomenon may be attributed to the fact that the particles, when they are charged with moisture or static electricity, or again when they are subject to softening which, even when limited, makes them sticky, adhere to the surface with which they are brought into contact. It is undesirable for excessive quantities of pigment particles to be deposited on the sensitized zones of the image carrier because, when this pigment is transferred to the recipient sheet of paper, there is a danger that the pigment which has been deposited in the configuration of the image formed by these sensitized zones will spread and thus blur the image. In addition, it is also undesirable for pigment particles to be deposited outside the sensitized zones of the image carrier owing to the fact that these particles, when transferred to the paper, form a background which reduces the contrast between the transferred image and the original background formed by the paper.
To remove the excess pigment from the surface of the image carrier, use was made in the prior art of an arrangement in which a mass of pigment particles, which was brought into contact with the surface of the image carrier downstream of the arrangement for applying the particles, removed the charge from the particles which were adhering to the carrier outside the sensitized zones, and the discharged particles then detached themselves from the image carrier and joined up with the mass. Nevertheless, such an arrangement is not entirely satisfactory in use owing to the fact that it does not always insure that the particles are fully discharged and thus does not enable the particles of pigment which are present on the image carrier outside its sensitized zones to be reliably removed. Furthermore, such an arrangement cannot be used to remove excess particles from the image carrier of an electrostatic printer since, when it picks up the electrical charges from the carrier, there is a danger that it will erase the sensitized zones on the carrier.
The present invention overcomes these disadvantages and provides a device which makes it possible effectively to remove excess pigment from the surface of the image carrier of a non-impact printing machine, which machine may be either of the electrostatic or magnetic type.